1. Field of the Invention
The present invention relates to a ventilation type optical recording disc, more particularly, to a ventilation type optical recording disc comprising one or more ventilation paths, through which are outside of the optical recording disc passes into the inside air gap of the optical recording disc.
2. Description of the Prior Art
In general, an optical recording disc has a well known sandwich construction, that is, in the optical recording disc, a pair of disc substrates are bonded together through an inner spacer and an outer spacer, so that each of recording layers of a pair of the disc substrates, which is formed on the surface of the disc substrate, faces together. In the optical recording disc, a space, which is called "the air gap" below, is formed between the recording layers of both of the disc substrates, wherein the air gap is isolated from the outside of the optical recording disc.
The optical recording disc comprising the isolated air gap can prevent an alien substance such as dust etc. from adhering to the recording layer of the disc substrate, because the substance can not enter the air gap, thereby information can be recorded correctly on the recording layer. However, in the aforementioned optical recording disc, if the environment condition of the optical recording disc such as the temperature or the atmospheric pressure changes, there will be a pressure difference between the air gap and the outside of the optical recording disc, and it will be difficult to track and focus on a predetermined tracking line of the recording layer. Moreover, the disc substrate may be broken when the shape of the disc substrate changes.
In order to prevent the disc substrate from breaking and to prevent the shape of the disc substrate from changing, as shown in FIGS. 1 and 2, an optical recording disc comprising a ventilation path was proposed in the Japanese patent provisional publications Nos. 106204/1979 and 171049/1984, wherein the ventilation path was provided to make the air pass through between the inside air gap and outside of the optical recording disc.
In the proposed optical recording disc, an air gap is formed between a pair of recording layers 21a and 22a formed on the surfaces of disc substrates 21 and 22, a ventilation path 26 is formed between the surface of the disc substrate 21 and the surface of an inner spacer 24 in an annular shape, and a ventilation path 27 is formed between the surface of the disc substrate 21 and the surface of an outer spacer 25 in an annular shape. A space is formed so that the space extends from the air gap through the ventilation paths 26 and 27 to the outside of the optical recording disc. There are provided filters 28, for preventing an alien substrance such as dust etc. from entering the air gap, at the position where the ventilation paths 26 and 27 are formed and on the outer wall surface of the inner spacer 24 and the inner wall surface of the outer spacer 25. There will not be a pressure difference between the inside air gap and the outside of the optical recording disc, even if the environment condition around the optical recording disc such as the temperature or the atmospheric pressure etc. changes, because the air in the inside air gap can pass through the ventilation paths 26 and 27 into the outside of the optical recording disc and reversely. This optical recording disc can overcome the aforementioned problems when the environment condition changes, and can prevent the alien substance such as dust etc. from entering the inside air gap and information can be recorded correctly on the recording layers 21a and 22a, because the alien substance is filtered.
However, the conventional ventilation type optical recording disc comprising the filters 28 has the disadvantage of having additional assembling members and having additional process steps in its manufacture than a ventilation type of optical recording disc does not comprise filter 28. Also this disc has the disadvantage that it is difficult to deal with the very small filters 28. Therefore, the optical recording disc comprising the filters 28 increases the cost of making the optical recording disc. Moreover, the filters 28 and alien easily removed from the wall surface of the inner and outer spacers 24 and 25 by a centrifugal force when rotating the optical recording disc and a strong air flow, when a large pressure difference or a large temperature difference between inside air gap 23 and outside of the optical recording disc occurs, thereby recording layers 21a and 22a could easily be damaged by the removal of filters 28 and substances such as dust etc. could entered from the outside of the optical recording disc through venitlation paths 26 and 27 into the inside air gap.